The present invention relates to an ink jet head, a control method for the ink jet head, and an ink jet recording apparatus with which an image is formed on a recording medium by ejecting ink toward the recording medium. More specifically, the present invention relates to an ink jet head, a control method for the ink jet head, and an ink jet recording apparatus with which ink containing charged fine particles are ejected through the ink jet head by utilizing electrostatic forces.
An electrostatic ink jet recording system is known in which ink is ejected toward a recording medium by utilizing electrostatic forces. In the electrostatic ink jet recording system, ink containing a charged fine particle component is used and ink droplets are ejected from ejection ports for ejection of the ink toward a recording medium by exerting electrostatic forces on the ink at the ejection ports through application of drive voltages to ejection electrodes arranged on the periphery of the ejection ports. By controlling the drive voltages applied to the ejection electrodes in accordance with image data, it is possible to record an image corresponding to the image data on the recording medium.
As an ink jet head used in a recording apparatus adopting the electrostatic ink jet recording system, a multi-channel head is known in which multiple ejection ports (channels) are arranged in one head. In order to perform recording at higher resolution using the multi-channel electrostatic ink jet head, it is required to dispose ejection portions at a high density and to control ejection ports independently of one another. With the electrostatic ink jet head as described above, however, ink droplets are ejected by utilizing electrostatic forces generated through application of voltages to ejection electrodes at the respective ejection ports. Therefore, when the ejection ports are disposed at a high density, electric field interference occurs between adjacent ejection ports and variations occur to the size of ejected ink droplets and to the flying direction of the ink droplets, which leads to a problem in that it is impossible to perform precise recording.
In view of such a problem, JP 2000-25233 A discloses an ink jet recording apparatus in which multiple ejection electrodes (individual electrodes) are provided on a substrate dividing ink guides and ink droplets are ejected from the ink guides by means of electrostatic forces generated through application of voltages to the ejection electrodes, where a shield electrode for shielding electric lines of force from adjacent channels is formed between the ejection electrodes. In this ink jet recording apparatus, a voltage, which is lower than the voltages applied to the ejection electrodes but is sufficiently high enough not to be discharged between the ejection electrodes, is applied to the shield electrode, thereby suppressing interference between adjacent ejection electrodes.
In the ink jet recording apparatus disclosed in JP 2000-25233 A, however, the ejection electrodes and the shield electrode are provided on the same surface, so it is impossible to shield electric lines of force generated from end portions on an outer peripheral side of the ejection electrodes with the shield electrode, which leads to a problem in that it is impossible to effectively prevent electric field interference between adjacent ejection ports.
In order to suppress the electric field interference, it is effective to provide a wide shield electrode between adjacent ejection ports. As in the case disclosed in JP 2000-25233 A in which the shield electrode and the ejection electrodes are provided on the same surface, however, when ejection ports are disposed at a high density, it is impossible to secure a sufficient width of the shield electrode between the adjacent ejection ports. On the other hand, when the intervals between the ejection portions are increased in order to increase the width of the shield electrode between the ejection ports, this results in an unfavorable situation in which the density of the ejection ports is lowered and a head size is increased. Therefore, there arises a problem in that it is difficult to perform precise recording with a compact head.
In order to draw a high-quality image at high speed on recording medium using an electrostatic ink jet head, it is required to supply a sufficient amount of charged colorant particles to ejection portions swiftly. As a method of supplying the colorant particles to the ejection portions swiftly, for instance, a method utilizing a liquid current and a method with which the colorant particles are caused to move to ejection ports by electrophoresis are conceivable.
These methods, however, are not enough to completely preclude a danger that an inconvenient situation will occur in which, for instance, a sufficient amount of colorant particles is not supplied to ejection portions swiftly, clogging of ejection ports of an ink jet head occurs, or dots formed on a recording medium are split.